JP2020157326A - Joining method of lead free solder material - Google Patents

Abstract

To provide a joining method and a solder joining part having high joining reliability even under a high temperature environment for forming a joining layer easily joinable, extremely small in the number of generation of voids after joining, uniform in generation of an intermetallic compound such as Cu6Sn5 and distributed in the whole joining part.SOLUTION: In a solder joining method for solder-joining by using a lead free solder material for generating a Cu6Sn5 intermetallic compound in a joining part after coating a soldering part of a base board of becoming previously a joining object by using Sn or a lead free solder alloy containing Sn as a main component, a solder joining part soldered by using the solder joining method can provide a highly reliable solder joining part small in reduction of a joining characteristic even when exposed for a long time at a high temperature of 300°C since the Cu6Sn5 intermetallic compound uniformly and abundantly exists in the solder joining part.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for joining a lead-free solder material utilizing liquid phase diffusion bonding.

Sn-Ag-Cu-based lead-free solder alloys and Sn-Cu-Ni-based lead-free solder alloys are widely used as bonding materials for electronic components in order to reduce the burden on the global environment.
However, most of these lead-free solder alloys have a melting point of 230 ° C. or lower, and there is a demand for a lead-free solder bonding material that replaces high-temperature solder containing a high concentration of lead, which is exempt from the RoHS Directive.
As a bonding material satisfying such needs, a sintering type bonding material containing silver particles as a main component and a liquid phase diffusion bonding material in which a metal powder and a solder alloy powder are mixed have been proposed.
However, the former is expensive and has not been widely used, and the latter also has various problems such as the generation of voids.
Therefore, a solder material that is inexpensive and has highly reliable bonding characteristics in a high temperature environment has been proposed.

For example, Patent Document 1 discloses a technique relating to a solder material using a solder having a relatively low melting point or a mixture of a solder powder having a relatively high melting point and a Ni-containing powder having a relatively high melting point.
Further, Patent Document 2 discloses a technique relating to a multilayer preform sheet containing Sn or a Sn alloy as a main material and an intermetallic compound of Cu and Sn.

The techniques disclosed in Patent Document 1 and Patent Document 2 are lead-free solder bonding materials having high temperature resistance by forming a Cu6Sn5 intermetallic compound in the bonded portion after bonding.
However, the techniques disclosed in Patent Document 1 and Patent Document 2 are not only costly but also fine because it is necessary to process a part or all of the bonding material into a single-layer or multi-layer preform sheet. There remains the problem that it is difficult to mount the surface on a joint having a complicated shape.

US2017 / 09591A1 Publication Japanese Patent No. 6042577

INDUSTRIAL APPLICABILITY According to the present invention, bonding is possible easily, voids are extremely small after solder bonding, and a bonding layer is formed in which intermetallic compounds such as Cu ₆ Sn ₅ are uniformly produced and distributed throughout the bonding portion. It is an object of the present invention to provide a solder joining method having high joining reliability even below, and a joint portion soldered by the joining method.

In order to achieve the above object, the present inventor has focused on the conditions for forming a Cu ₆ Sn ₅ intermetallic compound in the solder joint, and as a result of repeated studies, Sn or Sn or is previously applied to the joint of the substrate or the like to be joined. By coating with lead-free solder containing Sn as the main component and then soldering using a lead-free solder material that produces a Cu ₆ Sn ₅ intermetallic compound, no voids are generated in the bonding layer and the Cu ₆ Sn ₅ metal We have found that the intermetallic compound is uniformly and totally produced, and have completed the present invention.

That is, in the present invention, the soldered portion of the substrate or the like to be bonded is previously coated with Sn or a lead-free solder alloy containing Sn as a main component, and then the lead-free solder that produces a Cu ₆ Sn ₅ metal-to-metal compound is formed on the bonded portion. This is a solder joining method in which solder joining is performed using a material. In the solder joint portion soldered using the solder joining method, Cu ₆ Sn ₅ metal-to-metal compounds are uniformly and abundantly present in the solder joint portion. It is possible to provide a highly reliable solder joint portion in which the joint characteristics are not deteriorated even when exposed to a high temperature of 300 ° C. for a long time.

Since the present invention can supply solder joints having excellent high temperature characteristics and solder joints at low cost by a simple method, it can be expected to be applied to in-vehicle power devices and the like that require high temperature resistance.

An external photograph of a solder material bonded to a Cu substrate by a conventional method. A cross-sectional photograph of a joint in which a solder material is bonded to a Cu substrate by a conventional method. A cross-sectional photograph of a joint portion in which a solder material is bonded to a Cu substrate by the soldering method of the present invention.

INDUSTRIAL APPLICABILITY The present invention is a lead-free solder material that produces a Cu ₆ Sn ₅ metal-to-metal compound on a joint after coating the soldered part of a substrate or the like to be joined with a lead-free solder alloy containing Sn or Sn as a main component in advance. This is a solder joining method in which solder is joined using.
As a joining step, the first step is to precoat the joint portion of the substrate or the like to be joined with Sn or a lead-free solder alloy containing Sn as a main component.
Then, as the second step, a lead-free solder bonding material that produces Cu ₆ Sn ₅ is applied on the solder joint portion coated with Sn or a lead-free solder alloy containing Sn as a main component on the substrate or the like in the first step. After that, it is a joining method in which the solder is joined by heating in a reflow furnace or the like.

The coating component used in the solder joining method of the present invention is a lead-free solder alloy containing Sn or Sn as a main component, and is not particularly limited as long as it has the effect of the present invention, but Sn alone is more preferable.
As a coating method, a plating method can be exemplified, but vapor deposition, sputtering, or the like may be used, but the thickness of Sn or a lead-free solder alloy containing Sn as a main component to be coated is preferably 10 μm or more.
Further, the substrate used for the solder bonding of the present invention is not particularly limited as long as the effect of the present invention is obtained, and the material is a ceramic such as aluminum nitride or SiC plated with Cu, Ni, Au or the like. However, the material may be a Cu lead frame or the like.

The lead-free solder material that produces a Cu ₆ Sn ₅ intermetallic compound at the joint portion used in the solder joining method of the present invention is not particularly limited as long as it has the effect of the present invention.
For example, an alloy composed of Cu and Ni and a solder alloy composed of Sn—Cu—Ni can be exemplified, and alloys and solder powders having different compositions may be used in the future.
The particle size is also not particularly limited as long as it has the effect of the present invention, and is preferably 1 to 35 μm.
Further, in the lead-free solder material that produces a Cu ₆ Sn ₅ intermetallic compound at the joint, a flux composed of a resin component such as rosin, a thixo agent, an activator, a solvent, etc. is added to the alloy powder or the solder alloy powder. A lead-free solder material processed into a solder paste is preferable, and a solder paste using a flux composed of an activator and a solvent (including a binder) that does not leave a residue after soldering is more preferable.

Next, an embodiment of the present invention will be described.
As the first step of the solder joining method of the present invention, Sn was coated on the substrate by the following procedure.
(A) Sample / substrate: Oxygen-free copper plate (20 x 20 x 0.5 mm)
-Coating material: 99.9% Sn
-Flux: NS-334 (manufactured by Nippon Superior Co., Ltd.)
-Masking material: Kapton tape (50 μm)
(B) Equipment-Hot plate (c) Procedure (1) Cover the circumference of the copper plate with Kapton tape so that a space of 5 x 5 mm is created in the center as the plated part.
(2) Add a small amount of flux to a copper plate whose circumference is covered with Kapton tape, and place it on a hot plate heated to 250 ° C.
(3) Immediately after the flux evaporates, 99.9% Sn 0.019 g is placed on an unmasked copper plate, and it is confirmed that Sn is melted and the copper plate is Sn-plated.
(Sn 0.019 g is a weight set so that the thickness becomes 100 μm when the plating area is uniformly plated in 5 × 5 mm.)
(4) When the plating is confirmed, the copper plate is removed from the hot plate, cooled, and the capton tape is peeled off to prepare a Sn-coated sample, which is the first step.
It was confirmed that Sn plating having a thickness of about 100 μm was coated on the copper plate.

As the second step of the solder bonding method of the present invention, the lead-free solder material that produces a Cu ₆ Sn ₅ intermetallic compound at the joint was solder-bonded using the solder paste having the composition shown in Table 1.

The solder paste shown in Table 1 was prepared by the following procedure.
(1) The powders 1 and 2 are uniformly mixed in a blending amount ratio.
(2) Mix 3 to 5 in advance and mix with (1) in a uniformly dissolved state, and mix uniformly.
Use as a solder paste.

The second step of the solder joining method of the present invention was carried out by the following procedure.
(1) As Example 1, a small amount of the solder paste shown in Table 1 is applied onto the Sn plating of the Sn-plated copper plate produced in step 1.
As Comparative Example 1, a small amount of the solder paste shown in Table 1 is applied onto a copper plate that is not Sn-plated.
(2) Place (1) on a hot plate heated to 250 ° C, check that the copper plate reaches about 230 ° C, heat for 3 minutes, and solder.
(3) (2) is taken out from the hot plate and cooled at room temperature to prepare a bonded sample of Example 1 and Comparative Example 1.

FIG. 1 shows an external photograph of Comparative Example 1 after solder paste bonding.
In FIG. 1, it seems that the solder 2 is bonded on the copper plate 1, but it is not actually bonded, and when the substrate is erected, the solder is peeled off from the copper plate.
Next, for Comparative Example 1, a solder in a state where the solder is not detached from the copper plate was prepared, and the result of observing the cross section thereof is shown in FIG.
An intermetallic compound 3 was bonded to the copper plate 11, but it was found that voids 4 were formed on the copper plate 11 and could not be bonded.
Further, although the solder layer 21 is present above the voids, particulate matter that seems to be solder powder or metal powder is observed, and no intermetallic compound is found in the entire joint layer.

On the other hand, in FIG. 3, which shows the joining cross section of Example 1 using the solder joining method of the present invention, the intermetallic compound 3 is uniformly formed on the copper plate 11 without any gap at the joining interface, and the solder layer 21 at the joining portion is formed. Intermetallic compounds are uniformly found in the entire bonding layer.
Further, when the intermetallic compound shown in FIG. 3-3 and the intermetallic compound widely distributed in the solder layer 21 were examined, it was found to be a Cu ₆ Sn ₅ intermetallic compound.

From this, in the solder bonding using the lead-free solder material that produces the Cu ₆ Sn ₅ intermetallic compound by the solder bonding method of the present invention, the high melting point Cu ₆ Sn ₅ intermetallic compound is formed at the bonding interface without the generation of voids. It was found that solder bonding with high heat resistance and high bonding reliability is possible because it is not only produced but also formed uniformly and on one surface in the bonding layer.

As described above, the solder bonding method of the present invention can use lead-free solder materials and metal particles, which are extremely inexpensive compared to silver particles, and is a simple method for solder bonding and solder bonding having excellent high temperature characteristics. Since it is possible to supply parts, it can be expected to be applied to in-vehicle power devices that require high temperature resistance.

1 Copper plate 11 Copper plate 2 Solder 21 Solder layer 3 Intermetallic compound 4 Void

Claims (2)

In the method of performing solder bonding using liquid phase diffusion bonding, a method of coating a substrate with Sn or a lead-free solder alloy containing Sn as a main component is used as the first step, and as the second step, in the first step. A solder bonding method comprising a method of applying a lead-free solder material produced by a Cu ₆ Sn ₅ intermetallic compound to a coated substrate. A solder joint portion characterized by being soldered by the method of claim 1.

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